It is known that thin films of an amorphous alloy j comprising at least one transition metal such as iron and cobalt and at least one rare earth element such as terbium (Tb) and gadolinium (Gb) have an easy axis of magnetization perpendicular to the film face and are capable of forming a small inverse magnetic domain with magnetization anti-parallel to the magnetization of the film. By corresponding the existence or non-existence of this inverse magnetic domain to "1" or "37 0", it becomes possible to record a digital signal on the amorphous alloy thin film as mentioned above.
As thin films of an amorphous alloy comprising at least one transition metal and at least one rare earth element which can be used as magnetooptical recording media, thin films of amorphous alloys of Tb-Fe series containing from 15 to 30 atom % of Tb, are disclosed, for example, in Japanese Patent Publication 57-20691. There are also known magnetooptical recording media comprising a thin film of an amorphous alloy of Tb-Fe series to which a third metal has been added. Magnetooptical recording media of Tb-Co series and Tb-Fe-Co series are known as well.
Though the magnetooptical recording media comprising a thin film of an amorphous alloy as exemplified above have excellent recording and reproducing characteristics, they still involve such a serious problem from a practical standpoint that the amorphous alloy thin film is subject to oxidation in the course of ordinary use and the characteristics come to change with time.
The mechanism of oxidative deterioration of amorphous alloy thin films comprising transition metals and rare earth elements as mentioned above, is discussed, for example, in Journal of the Society of Applied Magnetism of Japan, Vol. 9, No. 2, pp 93-96, and this paper reports that this mechanism of oxidative deterioration may be classified into three types as noted below.
(a) Pit corrosion
By pit corrosion is meant the occurrence of pinholes in the amorphous alloy film. This corrosion proceeds mainly under the circumstances of high humidity, and it markedly proceeds, for example, in films of Tb-Fe and Tb-Co series.
(b) Surface oxidation
A surface oxide layer is formed on the surface of the amorphous alloy film, whereby the Kerr-rotation angle .theta.k of the film changes with time and eventually comes to decrease.
(c) Selective oxidation of rare earth element
Rare earth elements present in magnetooptical recording films are selectively oxidized, whereby coercive force Hc of the films comes to largely change with time.
Various attempts have heretofore been made to inhibit such oxidative deterioration of amorphous alloy thin films as mentioned above. For instance, there is proposed a procedure in which a thin film of an amorphous alloy is to have a three-layer structure wherein the film is sandwiched between anti-oxidizing protective layers such as those of Si.sub.3 N.sub.4, SiO, SiO.sub.2, and AIN. The anti-oxidizing protective layers as proposed above, however, involved such problems that they are relatively expensive and, at the same, time, they require much time and labor to be formed on amorphous alloy thin films, and that a sufficient inhibition of oxidative deteroration of the films is not always expected even when such anti-oxidizing protective layers are formed on the films.
Furthermore, various attempts are being made to improve resistance to oxidation of amorphous alloy thin films by incorporating a third component metal into the films such as Tb-Fe and Tb-Co series.
For instance, Journal of the Society of Applied Magnetism of Japan cited above discloses an attempt to improve resistance to oxidation of amorphous alloy thin film of Tb-Fe or Tb-Co series by incorporation into the films of such third component metal as Co, Ni, Pt, Al, Cr and Ti in an amount of up to 3.5 atom %. In connection with the attempt, the said Journal reports that the incorporation of small amounts of Co, Ni and Pt into Tb-Fe or Tb-Co is effective in inhibiting the surface oxidation and hole corrosion of the resulting film but has not effect on inhibition of the selective oxidation of Tb contained as a rare earth element in this film. This disclosure means that when small amounts of Co, Ni and Pt are added to Tb-Fe or Tb-Co, Tb present in the resulting film is selectively oxidized, and coercive force Hc of the film largely changes. Thus, even when small amounts up to 3.5 atom % of Co, Ni and Pt are added to Tb-Fe or Tb-Co, no sufficient improvement in resistance to oxidation of the resulting film is made.
With the view of improving resistance to oxidation of amorphous alloy thin films, a teaching on the amorphous alloy thin films which are obtained by adding Pt, Al, Cr and/or Ti in an amount up to 10 atom % to Tb-Fe or Tb-Fe-Co is disclosed on page 209 of the Proceedings of The Nineth Conference the Society Applied Magnetism of Japan (Nov. 1985). Even when Pt, Al, Cr and /or Ti in an amount up to 10 atom % are added to Tb-Fe or Tb-Fe-Co, however, inhibition of selective oxidation of Tb present in the resulting films is not sufficient, though the surface oxidation and hole corrosion can be inhibited to a fairly effective extent. Thus, there was still left such a problem that coercive force Hc of the resultant films will largely change with time, and eventually the coercive force Hc will largely decrease.
The prior art references discussed above do not disclose a thin film of an amorphous alloy as disclosed herein.
Journal of Magnetism & Magnetic Materials, 41, (1984), pp 128-130 and pp 125-127 discloses amorphous alloys of Fe-B series having added thereto up to about 3 atom % of Co, Cr or Pt. The amorphous alloys are formed into ribbons having a thickness of from 25 to 30 .mu.m, and their magnetic strains are studied. None of the disclosed alloys has, however, a sufficiently improved resistance to oxidation.
Furthermore, amorphous alloy ribbons obtained by a melt spinning or splat cooling method as described in this paper are magnetizable within the ribbon plane rather than perpendicularly to the ribbon plane. No consideration is given as to possibility of vertical magnetization of the ribbons.
Japanese Patent Laid-Open Publication 58-7806 discloses polycrystalline thin films having a composition of PtCo in which Pt is contained in an amount of 10-30 atom %.
However, the polycrystalline thin films having this composition of PtCo involves such problems that the polycrystalline thin films as formed require heat treatment such as annealing because that are polycrystalline, that grain boundaries sometimes appear as noise signals, and that the polycrystalline thin films are high in Curie point.